Quenched and tempered low-alloy steel

ABSTRACT

A hypoeutectoid boron steel product characterized by a minimum yield strength of 100,000 p.s.i. and a Charpy V-notch 15 ft.-lb. impact transition temperature below -50* F. in thick sections.

o United States Patent [151 3,645,723

Riedel Feb. 29, 1972 [54] QUENCHED AND TEMPERED LOW- 3,316,084 4/1961 naii aneual; .j ....7 5/12 w ALLOY STEEL 3,290,128 12/1966 Manganello ..75/l28W 3,290,183 12/1966 Ohtake ..75/l28 W [72] Invent Bethlehem 3,508,911 4/1970 Riedel ..75/l28 w [73] Assignee: Bethlehem Steel Corporation v [221 Wed Nov. 28 1969 Primary Examiner-Hyland Bizot Attorney-Joseph J. OKeefe [21] Appl. No.: 880,978

[57] ABSTRACT [52] U-S-Cl. ..75/l24, 75/128 F,75/lli/\;/6, A hypoeutectoid boron Steel product characterized y a [51] Int Cl (322C 39/20 C22C 39/02 minimum yield strength of 100,000 p.s.i. and a Charpy V- [58] Fie'ld 24 128 w T 128 B notch 15 ft.-lb. impact transition temperature below -50 F. in

................ thick sections [56] References Cited 4 Claims N Drawings UNITED STATES PATENTS BACKGROUND OF THE INVENTION This invention relates to a high strength, low alloy steel, and more particularly to a hypoeutectoid boron steel having the combination of a high yield strength and a low impact transition temperature.

In the past, steels having both a high yield strength and a low impact transition temperature have been produced by heat treating hypoeutectoid boron steels. However, this combination of properties could not be obtained in thick sections, for example, in plates from 2% to 6 inches thick.

Several approaches to the problem of producing thick sections having high yield strengths and low impact transition temperatures were made. One approach comprised adding to the steel relatively large amountsof elements which increase the hardenability of the steel. For example, plates from 2% to 6 inches thick having a minimum yield strength of 100,000 p.s.i. and a V Charpy impact transition temperature below 50 F. were produced from steel complying with ASTM specification A 543-67 and containing 3.50 wt.% nickel, 1.75 wt.% chromium, and 0.53 wt.% molybdenum. This approach, however, was not satisfactory in that the relatively large amounts of alloys were costly and detracted from the weldability of the steel.

Another approach comprised adding small amounts, e.g., 0.03 to 0.10 wt.%, of such elements as vanadium, titanium and columbium to hypoeutectoid boron steel. However, these elements are also costly and, while they impart strength and hardenability to the steel, they detract from impact properties so that an impact transition temperature below 50 F. cannot be obtained in combination with a minimum yield strength of 100,000 p.s.i. for a plate having a thickness of 2% inches or more.

It is an object of this invention to provide a low alloy boron steel characterized by the combination of a minimum yield strength of 100,000 p.s.i. and a V15 Charpy impact transition temperature below 50 F. It is a further object to provide such properties in both a steel plate from 2% to 6 inches thick and a round steel bar having a diameter of from 3 to 9 inches.

SUMMARY OF THE INVENTION I have discovered that the foregoing objects can be obtained by providing a hypoeutectoid boron steel a novel com- DESCRIPTION OF THE PREFERRED EMBODIMENT The steels of the invention consist essentially of:

balance essentially iron. By balance essentially iron," I do not wish to exclude residual elements or elements which are added during the steelmaking process which do not appreciably adversely affect the properties of the steel. For example, the steel may contain 0.035 wt.% max. phosphorus and 0.040 wt.% max. sulfur, although the amounts of these elements are preferably much lower, e.g., 0.020 wt.% max. phosphorus and 0.030 wt.% max. sulfur. As above mentioned, elements which detract from impact properties, e.g., vanadium and columbium, should be as low as possible. In no case should the sum of the vanadium, columbium and titanium exceed 0.025 wt.% max.

Steels of the above compositions should be produced by a steelmaking practice in which the silicon, aluminum, titanium and boron are sequentially added to a ladle into which a partially deoxidized steel is being poured. This practice is described in detail in application, Ser. No. 626,430, filed Mar. 28, 1967, by John Y. Riedel.

Following solidification, the steels may be rolled into shapes, e.g., plates or bars, in the usual manner. Said shapes must then be-heat treated to impart the combination of a high yield strength and a low impact transition temperature hereto. A suitable heat treatment comprises heating the steel shape uniformly to a temperature within the range of 1,625 to l,675 F quenching it in water, and tempering it within the range of l,l00 to l,200 F. for minutes per inch of thickness The microstructure of the steel following said heat treatment is predominantly tempered martensite.

The following Table 1 lists four steels which were rolled into shapes, heat treated and tested. The results of the tests are shown in Table II.

bination of the hardenability-imparting elements nickel, chromium and molybdenum, while restricting the quantity of those elements which detract from impact properties. More. specifically, l have found that the steel should contain 1.20 to 1.50 wt.% nickel, 0.85 to 1.20 wt.% chromium, and 0.45 to 0.65 wt.% molybdenum. In addition, titanium, which is added to the steel as a deoxidizer, should be present in an amount of from 0.01 to 0.025 wt.%. Elements such as vanadium and columbium are preferably absent but, if present, the sum of the vanadium, columbium and the titanium should in no case be above 0.025 wt.% max. Other elements, e.g., carbon, manganese, etc., should be held within their customary ranges for this type of steel.

Prior to testing, Steel A was heated to l,650 F., held at said temperature for 240 minutes, roller quenched in water, and cut into sections 4X8X6 inches. Said sections were tempered at 1, l75 F. for for 320 minutes. Steel B was similarly treated,

but was held at 1,650 F. for 360 minutes. Steels C and D, which were round bars, were uniformly heated to 1,650 E, roller quenched in water, and tempered at l,100 F. for 60 minutes per inch of thickness. These bars were tested for yield Strength a the s in r there anatmints hFL fLEXJEEWQ said center and the periphery thereof, i.e., at one-quarter thickness.

As can be seen, said steels A-D are characterized by the combination of yield strengths in excess of 100,000 p.s.i. and V Charpy transition temperatures below -50 F.

As a specific example of my invention, a 230 ton basic open hearth melt of the following composition was produced in the usual manner:

C M11. P S Si Ni C'r M0 wt. 0.16 0. 0 91s 0. 03s 0. 03 1. 22 0.10 0.44

The melt was blocked by first adding thereto 3,500 pounds of silicomanganese and one minute later adding 7,000 pounds of low carbon ferrochromium. The heat was then tapped from the furnace into a ladle containing 2,000 pounds of ferrosilicon (50 percent silicon).

As the heat was being tapped into the ladle, additions of carbon (coke), aluminum, titanium and ferroboron (13 percent boron) were made to the ladle in the named sequence. The ladle analysis of the steel was as follows:

CMnP 8 Si Ni CrMoAl Ti-B The steel was teemed into ingot molds and hot-topped. Subsequently the ingots were heated to 2,440-2,460 F. and rolled into slabs. The slabs were reheated to 2,3502,400 F. and rolled into plates 4 inches thick. The plates were finished at 1,650 F.- and water quenched. Subsequently, said plates were tempered at l,175 F. for 240 minutes. The plates had the following properties:

0.2% other V I5 Charpy Ten Y.S. (k.p.s.i.)

l 1 1,700 B5 Longitudinal l 1 1.000 Transverse 1 claim:

1. A low alloy, high strength steel consisting essentially of carbon 0.12 to 0.21 wt.%, manganese 0.45 to 0.70 wt.%, silicon 0.20 to 0.35 wt.%, nickel 1.20 to 1.50 wt.%, chromium 0.85 to 1.20 wt.%, molybdenum 0.45 to 0.65 wt.%, boron 0.001 to 0.005 wt.%, aluminum 0.01 to 0.15 wt.%, titanium 0.01 to 0.025 wt.%, balance essentially iron.

2. A quenched and tempered steel product of the composition claimed in claim 1, said steel having the combination of a minimum yield strength of 100,000 psi. and a Charpy V- notch 15 ft.-lb. impact transition temperature below -50 F.

3. A quenched and tempered plate of the steel recited in claim 1, said plate having a thickness of 2% to 6 inches, said steel having the combination of a minimum yield strength of 100,000 p.s.i. and a Charpy V-notch l5 ft.-lb. impact transition temperature below 50 F.

4. A quenched and tempered round bar of the steel recited in claim 1, said bar having a diameter of 3 to 9 inches, said steel having the combination of a minimum yield strength of 100,000 p.s.i. and a Charpy V-notch l5 ft.lb. impact transition temperature below 50 F.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 645 725 Dated February 29 a 1972 John Y. Riedel Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The term of this patent subsequent to April 28, 1987, has been disc'laimed.

Signed and Scaled this Arrest:

RUTH C. MASON C. MARSHALL DANN Altvstmg ()ffl'cer ('ummissl'mu'r uf Parents and Trademarks 

2. A quenched and tempered steel product of the composition claimed in claim 1, said steel having the combination of a minimum yield strength of 100,000 p.s.i. and a Charpy V-notch 15 ft.-lb. impact transition temperature below -50* F.
 3. A quenched and tempered plate of the steel recited in claim 1, said plate having a thickness of 2 1/2 to 6 inches, said steel having the combination of a minimum yield strength of 100, 000 p.s.i. and a Charpy V-notch 15 ft.-lb. impact transition temperature below -50* F.
 4. A quenched and tempered round bar of the steel recited in claim 1, said bar having a diameter of 3 to 9 inches, said steel having the combination of a minimum yield strength of 100,000 p.s.i. and a Charpy V-notch 15 ft.-lb. impact transition temperature below -50* F. 